The invention disclosed herein relates generally to electric motor actuators for condition changing means, such as dampers in air conditioning systems, and more particularly to backup energization apparatus for such actuators for returning the condition changing means to a predetermined state in the event of primary power failure.
In ventilating and air conditioning systems, it is common to utilize dampers and/or valves for controlling the temperature, humidity, and distribution of a fluid medium used in controlling conditions in an air conditioned space. Where air is used as the fluid medium, dampers may be used for mixing varying proportions of air at different temperatures and/or humidities within the air conditioning system, mixing air within the system with variable amounts of outside air for achieving operating economies, and distributing conditioned air to areas within the conditioned space.
The dampers and/or valves are frequently positioned by means of either single direction or reversible electric motors. The motors are powered from an available primary power source such as electrical current supplied by a public utility. This electrical power is supplied to the motors under the control of one or more condition responsive sensors, such as thermostats, enthalpy sensors, etc., so as to achieve the desired conditions in the air conditioned space, and to do so economically.
In installations employing a single-direction motor, the motor is used to move the damper or similar device toward one position, and a return spring is typically used to achieve movement in the opposite direction. Gearing on the motor is normally such that the spring is unable to return the damper as long as the motor is mechanically engaged. Thus, when it is desired to move the damper in a direction opposite that produced by motor operation, a clutch between the motor and the damper is disengaged to permit the spring to return the damper to a predetermined position.
An additional function which may be performed by the spring return apparatus is to position the damper at a predetermined desired position in the event of primary power failure. For example, it may be desirable or required to position an economizer damper which admits outside air to an air conditioning system to a predetermined minimum or closed position in the event of primary power failure. This may be necessary to prevent the admission of unacceptably cold outside air which could freeze water pipes and do other damage, and/or to minimize other adverse effects on conditions within the conditioned space.
In installations in which a reversible electric motor is employed, the damper may be moved in either direction under the power of the motor. However, even with a reversible motor, spring return apparatus is frequently employed to achieve appropriate positioning of the damper in the event of primary power failure.
As previously indicated, the use of a clutch in electric motor/return spring systems is necessary to mechanically disengage the motor so as to permit the spring to position the attached damper. A motor mechanism must be especially adapted for use with and to include a return spring and associated clutch. Therefore, in situations in which it is desired to replace an existing non-spring return unit with one having a return feature, it is necessary to replace the entire unit. Since it is not always known at the outset whether spring return operation will be required, and since it sometimes becomes necessary after the initial installation to add the return feature, it would be advantageous to be able to use the same motor actuator regardless of whether the return feature might be required, and to provide the return feature, if required, by a simple field addition.
An additional characteristic which is inherent in motor actuators with spring return mechanisms is that the motor must be sufficiently powerful in at least one direction to both operate the attached damper and to wind the return spring. This requires a motor which is larger and more expensive than necessary to achieve the basic purpose of positioning the damper.